A lithographic apparatus is a machine constructed to apply a desired pattern onto a substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). A lithographic apparatus may for example project a pattern from a patterning device (e.g. a mask) onto a layer of radiation-sensitive material (resist) provided on a substrate (e.g. a silicon wafer).
Before a pattern is projected from a patterning device onto a layer of radiation sensitive material provided on a substrate the topography of the substrate is measured. In order to achieve this, the lithographic apparatus is provided with a topography measurement system. The topography measurement system measures the height of the substrate across the surface of the substrate. The height measurements are used to form a height map which assists accurate projection of a pattern onto the substrate.
It may be desirable to provide, for example, a topography measurement system which obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.